I had not previously heard of it, but it sounded like a sugar and Google confirms that is exactly what it is.
It might have been one of those things where something is described as "low carb" because it's not as carby as something that's very high carb, but it's still pretty high carb itself. So inositol might not be digested to as much glucose as - say - sucrose, but still isn't something I'd want to be eating.
My view is that very little that you read on facebook or other social media is trustworthy, all the more so when they're trying to sell something..